In semiconductor device manufacturing, in particular, semiconductor wafers, it is often necessary to track single devices or lots through their sequence of processing steps. For this reason, each device is marked with a device identification number comprising, e.g., the lot name and the device number. In case of entering a processing tool, the device identification supplied on the device surface is read out in order to check, whether the correct device is currently being processed, and perhaps information is retrieved from a database, whether specific actions are to be taken for the current device.
In the case of semiconductor wafers as an example, single wafer tracking is enabled by engraving the wafer identification onto the wafer backside surface in, e.g., the form of a dot matrix by use of a laser. A laser spot engraves a hole at a specified position in the matrix on the device surface, e.g. representing a binary “one”. If a matrix element position is free of an engraved laser dot, a binary “zero” is provided, then. Thus, the binary dot matrix can be read out and be interpreted in terms of an identification number.
Alternatively, the matrix of dots directly corresponds to a structure providing a number or character when being recognized. Commonly, the device identification number is first read out by use of a CCD-camera, and second, identified by using a pattern recognition algorithm in an image processing unit.
In the same way, it is also possible to structure elevations on the device surface, e.g., with the help of a laser depositing some material at predefined matrix positions. The read-out process and the recognition is then performed in the same way as described above.
The approach using a CCD-camera has several disadvantages under productive circumstances. For example, the read-out rate of the engraved dot matrix can be too low originating from discoloration of the surface leading to ring-like structures on the surface layer of the semiconductor wafer. Such a ring running through a laser dot or hole can result in a non-detection of the corresponding matrix element, thereby issuing an error signal if the corresponding wrong wafer identification is currently not expected at the processing tool.
Moreover, dust particles due to previous processing may have deposited inside the engraved holes, thereby additionally decreasing the color contrast, which is necessary for the optical CCD-camera approach. Also, deposited oxide layers lead to a reduction of the depth of the holes obliterating the sidewall contrast for the CCD-camera. In such cases, only concave surfaces indicate the existence of underlying laser-engraved holes.
In order to circumvent these disadvantages, an apparatus using a laser emitting coherent light onto the laser dots or holes and a detector array receiving the light that is scattered at the concave sidewalls of laser holes being covered with an oxide layer is provided in DE 197 52 663 A1. There, a laser hole, or pattern, is recognized by an enhanced contrast due to the surface having plain, concave, or convex structure, thereby concentrating or diluting, respectively, the light scattered onto the array. Due to the enhanced contrast, a contrast-rich surface map of the wafer can be established by scanning the surface portion under investigation. The map of scattered light can then be evaluated into a potential map of the surface. Using an image processing unit with a pattern recognition algorithm, the wafer identification can then be performed.
Unfortunately, this approach becomes particularly advantageous only in the case of the laser holes being obliterated by deposition of an oxide layer. If alternatively the laser inscription itself is not deteriorated, but rings of discoloration only impose changes of surface characteristics, the scattering approach becomes inefficient. Moreover, efforts have to be spent in order to calculate the potentials of the laser dots, and in case of dense matrices, it is questionable whether a unique potential solution can be expected. In the case of steep sidewalls of the laser holes, the contrast even reduces due to the lack of convex or concave surface elements.